Cytometers, as an instrument for counting and classifying various cells, have been widely used in the medical and biological fields. Conventional cytometers typically employ a flow cytometry. Such a cytometer is comprised of a light source, a flow chamber and a photoelectric detection unit. The flow chamber provides an optical cell-interrogation zone, in which a sample flow of cells is encircled in a sheath flow according to the sheath flow principle (i.e., fluid focusing principle), so that the cells pass through the detection passage one by one. The light source, usually a laser, provides an illuminating light beam which may irradiate into the cell-interrogation zone of the flow chamber, such that the illuminating light beam may irradiate onto the cells (e.g., blood cells) flowing through the detection zone so as to be scattered, or excite fluorescence emission, etc. The photoelectric detection unit is useful for collecting various optical information generated in the flow chamber and converting it into electric signals. By processing and analyzing these converted electric signals, the parameters of various cells contained in the blood can be obtained in order for subsequent processing such as counting and classification, etc.
Normally, certain properties of cells are all represented by the peak or pulse width of the signals described above, and thus it is necessary to obtain such data as the peak or pulse width of various optical information. Some desired parameters concerning the blood cells may therefore be calculated by using a histogram or scattergram plotted with these data.
In prior art, for facilitating the blood cells passing through the optical cell-interrogation zone to be detected, the light beam should radiate into the interrogation zone of the flow chamber so as to form a spot, as shown in FIG. 1, the light beam LB, eradiated from a light source, is focused on the center of the cell-interrogation zone of the flow chamber 4 by an optical system, forming an elliptical spot BS on a plane π perpendicular to the optical axis at the center of the cell-interrogation zone. The minor axis of the spot should coincide with the flowing direction {circumflex over (f)} of the sample flow. Therefore a light beam shaping module is provided behind the light source, which is used for shaping the beam emitted from the light source, including collimating and converging the light beam. In prior art, light beams are converged by a normal spherical lens. However, in the case of this convergence, the energy distribution of the optical field converged at the cell-interrogation zone of the flow chamber is relatively narrow, with the typical value of 20 μm (minor axis)×80 μm (major axis). Moreover, the direction of the minor axis is the same as the flowing direction of the sample flow in the flow chamber, while the direction of the major axis is perpendicular to the plane defined by the flowing direction of the sample and the transmitting direction of the light beam. Such an energy distribution goes against the stability of signal detection and high-speed cell-interrogation. When the routes via which the cell particles flow through the interrogation zone become chaotic due to an increase in detection rate or other causes, the cell flow may deviate from the energy center of the focused spot, leading to a weakening of the photoelectrically detected signal. As a further result, the scattergram of the detected signals may be subjected to a false appearance, which may affect the accuracy of counting and classification.
Another problem with the prior art cytometer is as follows. In principle, an ordinary spherical Lens is adopted in prior art. Thereby, the collimation concerned is achieved by locating the light source at the focus of such a lens. However, the light beam has a very large angle of divergence in the direction of cell flow if the laser is positioned as shown in FIG. 1, and when the ordinary spherical lens is converging the light beam having a relatively large angle of divergence (non-adaxial) which is emitted from the points on the optical axis, it is not a dot that is converged onto the optical axis, but instead a blurry spot. In other words, light with a different height from the optical axis are converged to different locations, so a significant aberration occurs after the light beam passes through the optical system. As a result, in addition to a main spot BS0, the spot focused at the cell-interrogation zone of the flow chamber further has two symmetrical sidelobes BS1 and BS2, in which I denotes the inner wall of the flow chamber and O the outer wall of the flow chamber, as shown in FIG. 2. Thereby, the signal genenerated by the cells having passed through such an illuminated region will correspondingly comprise sidelobes P(1) and P(2) at both sides of the main pulse signal P. If the amplitudes of these pulses should also be identified as a scattered signal of the cells, the result of the sample detection would be incorrect. In a prior art approach for resolving this problem, a scattered pulse signal recognition algorithm is in principle incorporated in the signal processing unit. That is, a threshold value is made use of to get rid of an accompanying signal. Specifically, when a peak is recognized smaller than a threshold value, it is thereby deemed to be a false accompanying signal and gets rejected, as shown in FIG. 3. However, this approach still has a significant disadvantage in that the main pulse of some smaller cells is usually even smaller than the accompanying pulse of the bigger cells, and consequently the real scattered signal of the small cells would be rejected along with the accompanying pulse of the bigger cells by using the threshold value, which will bring error to the cell analysis.